Fracture tough ultra high strength steel sheets



Oct. 6, 1964 J. H. c-:Ross 3,152,020

F'RACTURE TOUGH ULTRA HIGH STRENGTH STEEL SHEETS Filed May 11. 1961 2 sheets-sheet 1 Uniax/'a/ sns//e Sfrengfn 9 me 1 Hydras/alie Burs/ Stress Un/.aX/.a/ weld Sheng, Hydrosfaf/c Y/e/d S/ress X- Fracture Tong/mess (Kc) E Dep/n of Decarburizalion, mi/s STEEL AIVD VESSEL NUMBER Comparison between rens/le properl/'es and nydroso/e fes/ properfies for mode/ pressure vessels TIF im Tf l s* EL i? I /NVENTOR JOHN H. GROSS Attorney Oct. 6, 1964 J. H. GROSS FRACTURE. TOUGH ULTRA HIGH STRENGTH STEEL SHEETS Filed May ll. 1961 2 Sheets-Sheet 2 4 8 /2 /6 20 Dept/7 of decarburizafion, mi/s Effecl of decarburizal/'on on burs/ sfress la y/e/d s/reng//I rafia /NVENTR JOHN H. GROSS Affomey United States Patent Olice 52,020 Patented Get. 6, 1954 3,152,020 FRACi llit TGUGH ULTRA HIGH STRENGTH STEEL SEETS .lohn H. Gross, Monroevilie, Pa., assigner to United States Steel Corporation, a corporatien of New .iersey Filed May 11, 1961, Ser. No. 109,286 4 Claims. (Ci. 143-36) This invention relates generally to high strength alloy steel sheets having high fracture toughness and pressure vessels formed therefrom.

1n aircraft, missile, and related industries there is a need for sheets of ultra-high strength steel that can be fabricated Without initiating cracks that propagate rapidly throughout the structure. Steel sheets are presently available that exhibit either suhicient yield strength or high fracture toughness; however, heretofore a substantial increase of one would result in a substantial decrease of the other. Expressed another Way, it has heretofore heen necessary to sacrifice fracture toughness to obtain yield strength. For example, Polaris-missile motor cases conventionally are fabricated from steel sheets with yield strengths approaching 200,000 p.s.i. The use of steels with yield strengths higher than 200,000 p.s.i. has not been heretofore feasible because steels with yield strengths higher than 200,000 p.s.i. have a fracture toughness which is too low.

It is, therefore, a principal object of this invention to provide a sheet of ahoy steel which has high yield strength and also high fracture toughness.

Yet another object of this invention is to provide a method of treating alloy steel sheets to produce surfaces which increase the fracture toughness Without substantially decreasing the yield strength of the steel.

Other objects and a fuller understanding may be had by reference to the following descriptions and claims taken in conjunction with the drawings, in which:

FIGURE 1 is a chart representing the results of var-ious tests performed on decarburized pressure vessels and test specimens;

FGURE 2 is a graph showing the effect of decarburization on hydrostatic burst-stress to uniaXial yield strength ratios;

FIGURE 3 is a side elevational view showing a pressure vessel formed according to this invention; and

FiGURE 4 is an enlarged sectional View taken along the plane designated by line 4 4 of FlGURE 3 showing substantial decarburization.

According to this invention, it has been found that surface decarhurization of ultra-high strength steels having a carbon content of at least 0.25% provides a substantial increase in fracture toughness of the steel, with very little corresponding decrease in yield strength. Pressure vessels, as shown in FGURE 3, were formed from sheets of steel varying in thickness from 0.072 inch to 0.095 inch. Four different steels were used in the fabrication of these pressure vessels. Five pressure vessels were formed from each of the steels having a composition shown in Table l.

TABLE I Chemical Composition of Steels Inveszgafed, Percent A more particular object of this invention is to provide a steel sheet having an outer surface thereon which promotes a high fracture toughness and which surface does not produce a corresponding reduction of the yield strength of the steel.

Another general object of this invention is to provide a pressure vessel which has a high yield strength and also a high fracture toughness.

The pressure vessel has a plurality of sheets Welded together to form a body 10. The body 10 includes an inwardly directed surface 11 and an outwardly directed surface 12. Reference characters 13 and 14 (FIGURE 4) indicate decarburized portions of the body 10 from the surfaces 11 and 12 respectively. The steel body of each vessel was decarburized to a depth from each surface, as shown in Table Il below.

TABLE Il Comparison of Sheet Material Properties and Hydrostatz'c Test Properties for Model Pressure Vessels Hydrostatie Smooth Tensile Properties Properties (Uni- Dlpth of axial) ecar- Steel Vessel burizao. tion, Yield mils Yield, Burst, Strength Tensile p.s.i. p.s.i. (0.2% Strength,

Oilset) p.s.i.

p.s.i.

X200- X1 18 237, 000 278, 000 215, 000 259, 000 X2 24 225, 000 242, 000 215, 000 259, 000 X3 24 226,000 250, 000 215,000 259, 000 X4 6 None 92,000 251,000 291, O00 X5 6 None 107,000 251,000 291, 001) 300M M1 16 240, 000 267, 000 216, 000 261, 000 Y M2 15 238, 000 276, 000 216, 000 261, 000 M3 16 24S, 000 274, 000 216, 000 261, 000 M4 10 None 154, 000 237, 000 280, 000 M5 10 None 187, 000 237, 000 280, 000 NBMC B1 18 226, 000 252, 000 204, 000 243, 000 B2 18 230, 000 252, 000 204, 000 243, 000 B3 20 238, 000 278, 000 204, 000 243, 000 B4 8 None 179, 000 234, 000 276, 000 B5 8 None 155, 000 234, 000 276, 000 4130 L1 17 192, 000 234, 000 186, 000 226, 000 L2 20 202, 000 252, 000 186, 000 226, 000 L3 20 199, 000 255, 000 186, 000 226, 000 L4 5 214, 000 224,000 173, 000 181, 000 L5 6 212, 000 218, 000 173, 000 181, 000

Ratio Fracture Toughness, Steel c, Hydro- Burst Burst p.s.i. Yield Stress Stress to Stress to in to Uuiaxial Uniaxial Uniaxial Yield Yield Tensile Strength Strength Strength X200 1 197, 000 1.10 1. 29 1.07 1 185, 000 1.04 1.13 0.94 1 157, 000 1. 05 1. 16 0. 97 89, 000 0.37 0.32 115, 000 0. 43 0. 37 300M 1 185, 000 1.11 1. 24 1.02 1 182, 000 1.10 1. 28 1. 06 1 164, 000 1. 15 1. 27 1.05 148, 000 0. 65 0. 55 140, 000 0. 79 0. 67 MBMC 1 231,000 1.11 1. 24 1.04 1 175,000 1.13 1. 24 1.04 1 222, 000 1. 16 1.36 1. 14 87, 000 0. 78 0. 65 74, 000 0. 66 0. 56 4130 1 302,000 1. 04 1. 26 1.04 1 285, 000 1. 08 1.35 1.11 1 296, 000 1.06 1.37 1.13 1 293, 000 1. 24 1. 30 1. 24 1 274, 000 1.22 1. 26 1. 20

1 Minimum value-sheet material plastieally strained during hydrostatic test.

The vessels were then heat treated to produce yield strengths indicated in Table Il. Separate specimens of each steel were also given identical treatment to that given each pressure vessel in order to obtain standard test results for each type of steel which would be independent of any defects which might occur in the forming of the vessels, and in order to produce data relating solely to the properties of the steel per se.

Each pressure Vessel was hydrostatically tested to failure. Each specimen was tested to determine its uniaxial yield strength, uniaxial tensile strength, and fracture toughness in the ambient temperature. The results of Vthese tests are given in Table Il and are also graphically plotted in FIGURE 1. High temperature testing was not performed.

Y Various ratios including hydrostatic yield strength to uniaXial yield strength, burst-stress to uniaxial yield strength, and burst-stress to uniaxial tensile strength are ncalculated and given in Table II. A graph showing the relationship of the effect of the depth of decarburization on the burst-stress to uniaxial yield strength ratio is shown in FlGURE 2. It will be noted that a rapid and substantial rise in the ratio of burst-stress to uniaxial yield strength occurs as decarburization increases up to a depth of about 0.016 inch. From about 0.016 inch to about 0.020 inch, the increase is less rapid. From about 0.020 inch to about 0.024 inch, there is some increase after which further depth of decarburization does not produce signiicantly increased ratios.

Steel 4130 was used in these tests to determine the elfect of decarburization on steels which have a yield strength of less than 200,000 p.s.i. Reference to Table Il and FIGURE 1 indicates that decarburized steels having yield strengths of less than 200,000 p.s.i. do not have their fracture toughness substantially increased by such decarburization. Hence, little or no beneiit is derived from decarburizing steel having a yield strength of less than 200,000 p.s.i. However, the other three steels, which have yield strengths greater than 200,000 p.s.i., have their fracture toughness substantially increased by decarburization with comparatively slight reduction in yield strength. This test data indicates that decarburization to a depth of at least 0.010 inch is necessary to pro duce sutiiciently high fracture toughness and that decarburization to a depth of more than 0.024 inch results in little additional benefit. Examination of Table 1I and FIGURES 1 and 2 indicates that the optimum depth of decarburization is at least 0.014 inch and not more than 0.020 inch.

According to this invention, decarburzation of the steel sheet to a depth of at least 0.010 inch and subsequent heat treatment to a uniaXial yield strength greater than 200,000 p.s.i. will produce a steel having a fracture toughness of at least 140,000 p.s.i.'vm

The pressure vessels tested, as indicated above, were formed of sheets having a thickness of between 0.072 inch and 0.095 inch. Thicker or thinner sheets decarburized would also exhibit similar increases in fracture toughness Without substantial reduction in tensile strength.

Also, deca-rburization of one surface of the steel rather than both surfaces Would result in a substantial increase in fracture toughness without a corresponding substantial decrease in tensile strength; however, the preferred treatment is to decarburize the steel from both surfaces.

While one embodiment of my invention has been shown and described it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

I claim:

l. Fracture tough ultra high strength steel alloy sheet adapted for fabrication into pressure Vessels, said sheet having a minimum thickness of about 0.072 inch, an ambient temperature uniaxial yield strength of at least 200,000 p.s.i., and a fracture toughness of at least 140,- 000 p.s.i. VT, the latter being imparted by decarburized surface having a depth of between 0.010 inch and 0.024 inch, the steel of said sheet beneath the decarburized surface having a carbon content of at least 0.25%, said carbon content being substantially higher than that of said surface, and said sheet having substantially between '5% and 7% of alloying elements, said alloying elements comprising 0.79% to 0.99% manganese, 0.009% to 0.016% phosphorus, 0.009% to 0.016% sulphur, 1.41% to 1.68% silicon, up to 1.72% nickel, 0.80% to 1.98% chromium, up to 0.40% molybdenum, and 0.05% to 0.11% Vanadium.

2. A pressure Vessel of steel alloy sheet of claim 1.

3. The alloy sheet of claim 1 in which the carbon content is substantially 0.40% and the decarburized surface depth is between 0.014 inch and 0.020 inch.

4. A pressure vessel of steel alloy sheet of claim 3.

(References on following page) 1961, pages 78-81. Published by the American Sociey for Meals, Mount Morris, 11i.

g References ited in the ie of this patent UNITED STATES PATENTS 2 241 369 Zieoler et al May 6 1941 Harold Berstein et al.: Metal Progress, v01. 78, No. 2, 679;466 Speldelsw ei May 25 :195 4 August 1969, pages 79-82. Published by the American 5 Society for Metals, Mount Morris, 111.

OTHER REFERENCES John M. Lynch: Metal Progress, v01. 79, No. 3, March 

1. FRACTURE TOUGH ULTRA HIGH STRENGTH STEEL ALLOY SHEET ADAPTED FOR FABRICATION INTO PRESSURE VESSELS, SAID SHEET HAVING A MINIMUM THICKNESS OF ABOUT 0.072 INCH, AN AMBIENT TERMPERATURE UNIAXIAL YIELD STRENGTH OF AT LEAST 200,000 P.S.I., AND A FRACTURE TOUGHNESS OF AT LEAST 140,000 P.S.I. $INCH, THE LATTER BEING IMPARTED BY DECARBURIZED SURFACE HAVING A DEPTH OF BETWEEN 0.010 INCH AND 0.024 INCH, THE STEEL OF SAID SHEET BENEATH THE DECARBURIZED SURFACE HAVING A CARBON CONTENT OF AT LEAST 0.25%, SAID CARBON CONTENT BEING SUBSTANTIALLY HIGHER THAN THAT OF SAID SURFACE, AND SAID SHEET HAVING SUBSTANTIALLY BETWEEN 3% AND 7% OF ALLOYING ELEMENTS, SAID ALLOYING ELEMENTS COMPRISING 0.79% TO 0.99% MANGANESE, 0.009% TO 0.016% PHOSPHORUS, 0.009% TO 0.016% SULPHUR, 1.41% TO 1.68% SILICON, UP TO 1.72% NICKEL, 0.80% TO 1.98% CHROMIUM, UP TO 0.40% MOLYBDENUM, AND 0.05% TO 0.11% VANADIUM. 